[patch, avr] Fix incorrect argument register for push dummy call

Sivanupandi, Pitchumani Pitchumani.Sivanupandi@atmel.com
Mon Feb 29 06:25:00 GMT 2016


On 26-02-2016 19:45, Luis Machado wrote:
> Hi,
>
> On 02/25/2016 09:34 AM, Pitchumani Sivanupandi wrote:
>> There are few failures for avr-gdb in callfuncs.exp. These failures are
>> due to the incorrect argument passing in case of push dummy call. The
>> last argument register is calculated incorrectly that caused this issue.
>>
>> Attached patch fixes the calculation so that the push dummy call aligns
>> with the code generated by avr-gcc.
>>
>> No new regressions found when tested with internal simulator.
>>
>> If ok, could someone commit please?
>>
>> Regards,
>> Pitchumani
>>
>> gdb/ChangeLog
>>
>> 2016-02-25  Pitchumani Sivanupandi<pitchumani.s@atmel.com>
>>
>>     * avr-tdep.c (AVR_LAST_ARG_REGNUM): Define.
>>     (avr_push_dummy_call): Correct last needed argument register.
>>
>>
>> avr-last-argument-register-fix.patch
>>
>>
>> diff --git a/gdb/avr-tdep.c b/gdb/avr-tdep.c
>> index 597cfb4..18ecfe4 100644
>> --- a/gdb/avr-tdep.c
>> +++ b/gdb/avr-tdep.c
>> @@ -111,6 +111,7 @@ enum
>>
>>     AVR_ARG1_REGNUM = 24,         /* Single byte argument */
>>     AVR_ARGN_REGNUM = 25,         /* Multi byte argments */
>> +  AVR_LAST_ARG_REGNUM = 8,      /* Last argument register */
>>
>>     AVR_RET1_REGNUM = 24,         /* Single byte return value */
>>     AVR_RETN_REGNUM = 25,         /* Multi byte return value */
>> @@ -1298,12 +1299,14 @@ avr_push_dummy_call (struct gdbarch *gdbarch,
>> struct value *function,
>>         const bfd_byte *contents = value_contents (arg);
>>         int len = TYPE_LENGTH (type);
>>
>> -      /* Calculate the potential last register needed.  */
>> -      last_regnum = regnum - (len + (len & 1));
>> +      /* Calculate the potential last register needed.
>> +         E.g. For length 2, registers regnum and regnum-1 (say 25 and
>> 24)
>
> Is that how the code works? It seems a bit confusing, because we want to
> start in even-aligned registers, so we'd use r25:r24, r23:r22 etc.

Yes. Lowest even register will be loaded with LSB of argument, and 
subsequent bytes passed in subsequent registers (i.e. in increasing 
register number).

> So last_regnum always ends up pointing at an odd-numbered register, so
> the code can decrement the odd-numbered register if needed (in case we
> have data of length 1/3/...).
>
>            /* Skip a register for odd length args.  */
>            if (len & 1)
>              regnum--;

No, last_regnum should point at last even register where LSB of argument 
should be loaded. Variable 'regnum' should point to register which loads 
MSB of argument. In case of odd length argument, regnum-1 will hold the 
MSB of argument. So, it is decremented.

Ref: https://gcc.gnu.org/wiki/avr-gcc#Calling_Convention

>> +         shall be used. So, last needed register will be
>> regnum-1(24).  */
>> +      last_regnum = regnum - (len + (len & 1)) + 1;
>>
>
> Isn't this going to be incorrect for, say, data of length 3?
>
> last_regnum = 25 - (3 + (3 & 1)) + 1 -> 25 - (3 + 1) + 1 -> 22
>
> But it should've been 21, because data would occupy registers 25:24 and
> 23:22 and then we'd need an even-aligned register to start with?

Variable last_regnum indicates the end of register that required to pass 
the argument. In case of length 3, we need 22 (LSB), 23 and 24(MSB) to 
pass the argument. So, last register required is 22, not 21.

Regards,
Pitchumani



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